Could you kindly help me with these 2 sums - Q 18 and Q21 - many, many thanks

I think one of the reasons no-one has attempted to answer question 18 is that the diagram is not clear. We can't see the graph of at all. This means that it's not possible to be sure what the value of is. However, here is some working and a diagram showing two possible solutions. You will have to take it from here.

First crosses the -axis where:

So, working in degrees:

See the black graph in the attached diagram.

So let's suppose first that this graph meets where . Then:

So that's one possible value of . See the yellow coloured graph.

But perhaps the diagram shows a different position of the graph, and they actually meet at a different point at which the first graph crosses the -axis; for instance, where . Then we get:

This is the pink graph.

Over to you. Can you sort it out now?

For question 21: multiply top-and-bottom by :

, if we are given that is acute, because is then positive.

But if is obtuse, and . So . So we would have to say:

Can you see what happens for other values of ? If you're not sure, look at the graphs of and , and see whereabouts and whereabouts . And then bear in mind that has to be positive.